FBE05 – Mathematics and Statistics Lecture 7 - Estimating

Introduction One of the important branches of building technology is to prepare the estimates of the cost involved in constructing a building. The overall cost of a project determines its viability and enables the client to arrange capital for financing the construction. The main components of the cost of constructing a building are the cost of materials, labour, plant and contractor’s profit. This chapter gives some examples on how to prepare approximate estimates. The costs of materials and labour are never constant and they also differ from one region to another. For latest information on the cost of materials, labour and plant reference should be made to publications which update this information at regular intervals.

Estimating Foundations Concrete is by far the most commonly used material in the construction of foundations of buildings. For plain concrete 1:3:6 concrete mix is usually used, but for reinforced concrete 1:2:4 or a stronger mix is used. The quantities of cement and aggregates for preparing a concrete mix may be determined by considering either their volume or mass. In this section we will consider how to work out the mass of the cement and the aggregates. This gives a better quality of concrete as we can take into account the moisture content of the aggregates if they are not dry.

Estimating Consider a 1:3:6 concrete mix. The density of concrete is approximately 2400 kg/m3 or in other words, 1 m3 of concrete has a mass of 2400 kg. To prepare 1 m3 of concrete the quantities of cement and the aggregates are:

Estimating The amount of water depends on where the concrete is to be used. For strip foundations the amount of water could be about 50% of the amount of cement, giving a water cement ratio of 0.5.

Estimating EXAMPLE Calculate the cost of materials and labour required to construct the 1:3:6 concrete strip foundation shown in Figure 6.1, given that: Cost of materials Cement = £3.60 per 25 kg bag Fine aggregates = £32.00 per 850 kg jumbo bag Coarse aggregates = £32.00 per 850 kg jumbo bag Labour Approximate labour hours = 2 per m3 for mixing and placing concrete Hourly rate = £12.00

Estimating